Long gone are the days when coatings fulfilled merely a decorative purpose—instead, they serve to protect the underlying substrates and incorporate diverse additional functions. It is therefore desirable to unite as many properties as possible in one coating.
The coating materials encompass, fundamentally, size colors, lime paints, varnishes, stains and glazes, emulsion paints, polymer-resin paints, and powder coatings. The coating materials used to produce coatings have already been much described in the prior art. Often employed in the coating materials are chemical compounds with reactive groups and composite particles, which crosslink on curing and produce coatings adhering to the substrate.
For instance, EP 2050796 B1 describes a coating with enhanced hiding power, compositions produced therefrom, and methods for production thereof, that are produced using composite particles. The method described in EP 2050796 B1 produces composite particles comprising a pigment particle and a plurality of polymer particles, with each of the polymer particles comprising at least one reacted complementary functional group which forms a covalent bond with the pigment particle. For this purpose, inorganic pigments are suspended in alcohols, and are functionalized by addition of long-chain monofunctional alkoxysilanes, and then polymer particles are added to the suspension, and are intended to produce a spacer effect and thereby to counteract the flocculation of the pigment particles. The functionalized particles are used in the form of a suspension, as a dispersion coating with a solvent content.
As well as solvent-containing systems there are also coating materials which are used in the form of “dry” mixtures. Powder coatings of this kind are likewise well known in the prior art. Generally speaking, these powder coatings comprise binders, additives, colorants, and fillers, but no solvents. The chemical differences between the raw materials used in powder coatings and in conventional coating materials are not great, and the crosslinking mechanisms of a powder coating film resemble those of a baking varnish, where two reactants form an organic network under the influence of temperature and thereby enter into a chemical union. Virtually all raw materials for powder coatings are present generally in the form of powder.
For reasons including primarily those of economics, fillers are used in powder coatings. The most frequently used fillers include calcium carbonate, heavy spar, and precipitated barium sulfate. A disadvantage of powder coating systems comprising filler is that they possess a poor hot water resistance. This can be attributed to the poor attachment of the binder to the filler surface. Water is able to penetrate into the microcracks that are present, and this may lead to altered optical properties, such as bleaching, for example, and even to poor corrosion resistance.
The pigments that are known in the prior art and are used in powder coatings are in some cases equipped with an organic aftertreatment, based for example on a polyalcohol, this aftertreatment being bonded only physically, via dipole-dipole bonds or van der Waals bonds, to the pigment surface. The primary contribution of these interactions is to improve wetting when the pigments are incorporated, but they do not contribute to steric stabilization. Such stabilization can be achieved, however, via the addition of suitable additives.
A disadvantage of the stabilizing components used hitherto, though, is that they tend to migrate. This leads to a deterioration in the optical properties (gloss, hue) and also in the service properties (e.g., corrosion resistance, hot water resistance).